A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Implantable medical devices may deliver electrical stimulation or fluid therapy and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some implantable medical devices include electrodes, and/or are coupled to electrodes via one or more implantable medical leads, for sensing intrinsic electrical signals within the patient and/or delivering electrical stimulation therapy to the patient.
Implantable medical devices, such as cardiac pacemakers or implantable cardioverter-defibrillators, for example, provide therapeutic electrical stimulation to the heart via implanted electrodes. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion or defibrillation. In some cases, an implantable medical device may sense intrinsic depolarizations of the heart, and control delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
Implantable pacemakers, cardioverters, defibrillators, or pacemaker-cardioverter-defibrillators are typically coupled to one or more intracardiac leads that carry electrodes for cardiac sensing and delivery of therapeutic stimulation. Subcutaneous devices, e.g., loop-recorders, which typically are not coupled to leads, have been used to monitor cardiac signals. Subcutaneously implantable medical devices may include a plurality of electrodes on or integrated with the housing of the device.
Additionally, subcutaneous devices and have been proposed for use in detecting of tachyarrhythmias and delivering responsive shocks. Such proposals have typically involved coupling a subcutaneously implanted medical device to one or more subcutaneously implanted electrodes via one or more implantable leads, although leadless, subcutaneous cardioverters of defibrillators have been proposed. Cardiac sensing and delivery of therapeutic shocks in such systems may be between a plurality of electrodes on the device housing, between an electrode on the device housing and a lead-borne electrode, or between lead-borne electrodes. It has also been proposed to deliver cardiac pacing pulses using such devices.
A cardiac signal sensed by an implantable medical device may include artifacts. In some cases, an implantable medical device may mistake an artifact for a cardiac depolarization. In some cases, the rate and duration of such artifacts may be such that an implantable medical device may interpret the artifacts to be a tachyarrhythmia.
Examples of an artifact that may be present in a sensed cardiac signal include motion artifacts, electromagnetic interference, or artifacts due to continuity or integrity issues with leads. Patient motion artifacts may be particularly evident in signals sensed via subcutaneous electrodes due to electromyographic signals generated by adjacent muscle groups within the patient. Limb and trunk movements or even breathing can generate noise spikes that are superimposed upon cardiac signal, and can make it appear to reflect a higher heart rate than the actual heart rate. Patient motion artifacts may also be present in signals sensed by implantable medical devices with intracardiac leads, such as when a unipolar electrode configuration involving a housing electrode is used for sensing, or whenever an electrode used for sensing is not in contact or in adequate contact with the heart.